🤹‍♀️ SELECT ORDER [SQL]

• Why can't use SUM() in WHERE?
• Why can't use AS in WHERE or GROUP BY?

😎 You will get the reason why it is like that

💎 Select order

When you see this SQL, what do you think about order?

SELECT * FROM clients 
WHERE    age >= 30
GROUP BY usa
HAVING   salary > 1000
ORDER BY id desc
LIMIT    100

Seems like sql executes from SELECT ?
actually it's not true

-- SQL executes in this order
FROM
|
WHERE
|
GROUP BY 
|
HAVING
|
SELECT
|
ORDER BY
|
LIMIT

💎 Question 1

Why SUM() function can't use in WHERE?

FROM
|
WHERE 👈 👈
|
GROUP BY 👈 👈
|
HAVING
|
SELECT
|
ORDER BY
|
LIMIT

Before WHERE executes, GROUP BY has not executed yet, so we can't use COUNT() SUM() in WHERE.

In other word, it's possible to use these functions in HAVING.

💎 Question 2

Why can't use AS in WHERE or GROUP BY? Even though in ORDER BY can use it, unfair!!

FROM
|
WHERE 👈 👈
|
GROUP BY 👈 👈
|
HAVING
|
SELECT 👈 👈
|
ORDER BY 👀 👀 
|
LIMIT

take it easy, man. Because ORDER BY executes after SELECT.
You can't read name which is created by "AS" before define it.

Thank you for reading 🤗